A varactor is an electrical device having a capacitance which is controlled by a suitable voltage or current bias. A varactor is used, for example, in Voltage Controlled Oscillators, VCOs, where a frequency of an oscillator is controlled by an applied voltage or current bias. VCOs are used, for example, when a variable frequency is required or when a signal needs to be synchronised to a reference signal. In radio communication devices, e.g. portable/cellular phones, VCOs are often used in Phase Locked Loop, PLL, circuits to generate suitable signals. Generation of a reference signal which is synchronised with a signal received by a radio receiver, modulation/demodulation operations and frequency synthesis are examples of such uses. Several varactors suitable for Integrated Circuit, IC, technologies are known in the prior art. R. A. Moline and G. F. Foxhall discuss in "Ion-Implanted Hyperabrupt Junction voltage Variable Capacitors", IEEE Trans. Electron Devices, ED-19, pp. 267f, 1972, the use of pn-diodes as varactors which may be used in bipolar, CMOS or BiCMOS technologies. It is also known to use Schottky diodes or MOS-diodes as varactors. The latter is described, for example, by S. M. Sze in "Physics of Semiconductor Devices" John Wiley & Sons, 2.sup.nd Edition, pp. 368f. The integration of the known varactors depends on the capability of the IC technology. An overview of the integrated devices for high frequency RF application in a BiCMOS process is given by J. N. Burghartz, M. Soyuer and K. Jenkins in "Integrated RF and Microwave Components in BiCMOS Technology", IEEE Trans. Electron Devices, Vol. 43, pp. 1559-1570, Sept. 1996. As is stated on page 1568 and in FIG. 12 the varactors are not a part of the standard BiCMOS device set. Instead it is proposed to use a collector-base junction of a bipolar transistor as a varactor. J. Craninckx and M. S. J. Steyaert suggest in "A 1.8-GHz Low-Phase-Noise CMOS VCO Using Optimized Hollow Spiral Inductors", IEEE J. Solid-State Circuits, Vol. 32, pp. 736-744, May 1997, the use of a p+/n-well junction diode as a varactor in a VCO which is integrated by means of a CMOS process.
Whilst the known forms of varactors described above functions quite adequately, they do have a number of disadvantages.
One drawback of the known varactors is that they are difficult to realise with high quality factors, Q, especially for high frequency applications in a conventional CMOS process due to their high series resistance or required additional manufacturing steps. This results in low yield and high manufacturing costs.
Another drawback of the known pn-junction varactors is that in many applications, such as when used in most VCO circuits, a DC de-coupling capacitor needs to be added to the design which makes the design even more difficult to integrate into an IC. The use of a DC de-coupling capacitor externally to the IC adds on to the overall cost of the implementation and consumes valuable space on a Printed Circuit Board, PCB. These drawbacks become even more pronounced in hand-held devices, such as portable phones, which need to be small and which are produced in high-volumes.
It is an object of the present invention to provide a varactor which overcomes or alleviates the above mentioned problems.